1. Field of the Invention
The present invention relates to a membrane switch for use in a keyboard device or the like.
2. Description of the Related Art
A conventional membrane switch will now be described with reference to FIGS. 9 to 12. As shown in FIG. 11, filmy upper sheet 21 and lower sheet 22 each made of an insulating material are formed integrally with each other through a connecting portion 23 and can be folded in two at the connecting portion 23.
The upper sheet 21 has a plurality of mounting holes 21a and the lower sheet 22 also has a plurality of mounting holes 22a formed in an opposed relation to the mounting holes 21a.
The upper sheet 21 is formed with a plurality of contact portions 24 using silver paste or the like and the lower sheet 22 is also formed with a plurality of contact portions 25 using silver paste or the like in an opposed relation to the contact portions 24. Each of the contact portions 24 and 25 is connected to each of connecting conductors 26 formed of silver paste or the like.
Further, the lower sheet 22 is formed with lead-out terminals 27 using silver paste or the like, with the connecting conductors 26 being connected to the lead-out terminals 27.
An insulating resist layer 28 is formed on the surface of the lower sheet 22 except the contact portions 25 and the lead-out terminals 27, as shown in FIG. 12.
In the insulating resist layer 28, one layer of insulating paste which contains a thermoplastic resin as a main component is formed on the surface of the lower sheet 22 by printing and the connecting portion 23 is folded, allowing the upper sheet 21 to be superimposed on the insulating resist layer 28. Thereafter, the upper and lower sheets 21, 22 are pressed vertically against each other under heating, with the result that both sheets are pasted together by the insulating resist layer 28.
In this laminated state, as shown in FIG. 9, the insulating resist layer 28 functions as a spacer so that the contact portions 24 and 25 are spaced from and opposed to each other and so that the mounting holes 21a and 22a are also opposed to each other.
A lower support member 29 is formed, for example, by a synthetic resin or metal plate and it has a plurality of holes 29a. The lower sheet 22 is put on the support member 29 in such a manner that the mounting holes 21a and 22a are opposed to the holes 29a.
A plate-like upper support member 30 is formed, for example, by molding a synthetic resin. The upper support member 30 has holes 30a formed oppsite to the contact portions 24 of the upper sheet 21 and also has columnar mounting portions 30b formed in an opposed relation to the holes 29a. The support member 30 is put on the upper sheet 21 with its mounting portions 30b inserted into the holes 29a, and end portions of the mounting portions 30b are pressure-deformed to unite the support member 29, lower sheet 22, upper sheet 21, and support member 30, thereby constituting a membrane switch.
The above membrane switch is attached, for example, to the case of a keyboard device so that the upper support member 30 is exposed to the surface of the case.
The membrane switch is operated in the following manner. The upper sheet 21 is depressed and deformed through the holes 30a of the support member 30, causing the contact portions 24 to come into contact with the contact portions 25 to switch ON. Further, the depressed and deformed state is released, allowing the contact portions 24 and 25 to be spaced from each other to switch OFF.
There also is known such a membrane switch as shown in FIG. 13, in which the support member 30 used in the above membrane switch is omitted for attaining the reduction in thickness, a support portion 29c of a lower support member 29 is projected to the upper surface side through holes 31 of upper and lower sheets 21, 22, an X-shaped leg portion 33 with a key top 32 attached thereto is secured to the support portion 29c, an operating portion 34 formed of a rubber material is disposed between the key top 32 and the upper sheet 21, and by depressing the key top 32 the membrane switch is operated through the operating portion 34.
In the conventional membrane switches described above, the insulating resist layer 28 formed as a single layer on the lower sheet 22 cannot be made uniform in thickness throughout the whole surface thereof, that is, the thickness thereof is non-uniform, so it is necessary to increase the pressing force against both upper and lower sheets 21, 22 for pasting the two together.
Further, even if the pressing force is increased, the laminated state is unsatisfactory at a thin portion of the insulating resist layer 28, which results in water soaking into the contact portions 24 and 25 through the unsatisfactory laminated portion from the mounting holes 21a and 22a which are exposed from the surface of the case, thus causing defective insulation, or water soaking into the contact portions 24 and 25 through the unsatisfactory laminated portion from the peripheral edge portions of the upper and lower sheets 21, 22, thus causing defective insulation.
Particularly, if the spacing between the contact portions 24, 25 and the mounting holes 21a, 22a is narrow, defective insulation is apt to occur in such a narrow portion.
The above conventional membrane switch has the problem that the insulating resist layer 28 formed as a single layer on the lower sheet 22 cannot be made uniform in thickness throughout the whole surface thereof and due to such a non-uniform thickness it is required to increase the pressing force against the upper and lower sheets 21, 22 for pasting the two together, thus resulting in increased elongation of both sheets.
Moreover, even if the pressing force is increased, the laminated state is unsatisfactory at a thin portion of the insulating resist layer 28, with the result that water soaks into the contact portions 24 and 25 from the mounting holes 21a and 22a which are exposed from the case surface, causing defective insulation, or water soaks into the contact portions 24 and 25 from the peripheral edge portions of the upper and lower sheets 21, 22, causing defective insulation. This is also a problem that is encountered in the conventional membrane switch.